a Germanic character of runic origin Þ used in Old and Modern Icelandic to represent the voiceless dental fricative sound of th, as in thin, bath. Its use in phonetics for the same purpose is now obsolete See theta

5.

this same character as used in Old and Middle English as an alternative to edh, but indistinguishable from it in function or sound Compare edh

6.

(zoology) any of various sharp spiny parts

7.

a source of irritation (esp in the phrases a thorn in one's sideorflesh)

Old English þorn "sharp point on a stem or branch," earlier "thorny tree or plant," from Proto-Germanic *thurnuz (cf. Old Saxon thorn, Dutch doorn, Old High German dorn, German Dorn, Old Norse þorn, Gothic þaurnus), from PIE *trnus (cf. Old Church Slavonic trunu "thorn," Sanskrit trnam "blade of grass," Greek ternax "stalk of the cactus," Irish trainin "blade of grass"), from *(s)ter-n- "thorny plant," from root *ster- "stiff."

Figurative sense of "anything which causes pain" is recorded from early 13c. (thorn in the flesh is from II Cor. xii:7). Also an Anglo-Saxon and Icelandic runic letter (þ), named for the word of which it was the initial.

(1.) Heb. hedek (Prov. 15:19), rendered "brier" in Micah 7:4. Some thorny plant, of the Solanum family, suitable for hedges. This is probably the so-called "apple of Sodom," which grows very abundantly in the Jordan valley. "It is a shrubby plant, from 3 to 5 feet high, with very branching stems, thickly clad with spines, like those of the English brier, with leaves very large and woolly on the under side, and thorny on the midriff." (2.) Heb. kotz (Gen. 3:18; Hos. 10:8), rendered _akantha_ by the LXX. In the New Testament this word _akantha_ is also rendered "thorns" (Matt. 7:16; 13:7; Heb. 6:8). The word seems to denote any thorny or prickly plant (Jer. 12:13). It has been identified with the Ononis spinosa by some. (3.) Heb. na'atzutz (Isa. 7:19; 55:13). This word has been interpreted as denoting the Zizyphus spina Christi, or the jujube-tree. It is supposed by some that the crown of thorns placed in wanton cruelty by the Roman soldiers on our Saviour's brow before his crucifixion was plaited of branches of this tree. It overruns a great part of the Jordan valley. It is sometimes called the lotus-tree. "The thorns are long and sharp and recurved, and often create a festering wound." It often grows to a great size. (See CROWN OF THORNS.) (4.) Heb. atad (Ps. 58:9) is rendered in the LXX. and Vulgate by Rhamnus, or Lycium Europoeum, a thorny shrub, which is common all over Palestine. From its resemblance to the box it is frequently called the box-thorn.